The use of power amplifiers for transmitting radio frequency signals has many applications, including, but not limited to, radiotelephone communications systems. When using a power amplifier in a radiotelephone certain qualities of the power amplifier are desirable including: 1) a large power control range, 2) temperature stability, 3) efficient use of current, and 4) minimal AM output.
A total power control range is limited on the high end by the gain of the amplifier at the maximum bias current and the power control range is limited on the low end by the isolation of the amplifier from the radio frequency (RF) input when the bias current is zero. Within this range, the output power is approximately proportional to the square of the bias current.
Typically, in order to obtain maximum gain and efficiency for a given bias current, class C amplifiers are used. In previous class C amplifier configurations, the isolation from the RF input, in absence of bias current, was limited to the parasitic capacitance between the collector and the base of the transistor.
In order to obtain the desired temperature stability in low efficiency amplifiers, a resistor is coupled between the emitter of the transistor contained within the amplifier to ground. This resistor stabilizes the amplifier over temperature, however, it is undesirable because it lowers the efficiency of the amplifier. In order to obtain high efficiency of the amplifier, the collector and emitter must be void of any lossy resistive elements. Removal of these lossy elements increases the potential gain and efficiency of the amplifier, however, it causes the amplifier circuit to be extremely temperature sensitive, unless other bias stabilization measures are employed.
FIG. 3 illustrates a commonly employed circuit topology for RF power amplifiers. The focus of the circuit is the bipolar transistor 309 in a common emitter configuration. This circuit attempts to attain bias control and temperature stability by using the clamping diode 305. The intrinsic characteristics of this clamping diode 305 are matched to the base emitter junction of the transistor 309. The diode voltage tends to track the base to emitter voltage of the transistor 309 resulting in a temperature stable bias current which is approximately proportional to the control voltage 301. The clamping diode 305 also serves as a circuit protector from excessive control voltage 301.
The amplifier of FIG. 3 would have a typical control characteristic and AM characteristic as illustrated in FIG. 5. The control characteristic is a plot of the RF output power 501 over the control voltage range 301. The AM characteristic is a plot of percentage AM output 503 for one percent AM input on the RF input signal over the control voltage range 301. The peak of the AM characteristic 503 tends to be aligned with the steep portion of the control characteristic 501. The steep portion may also be defined as the point of maximum slope.
If two amplifiers as shown in FIG. 3 are cascaded and the control voltage inputs are tied directly together, then the composite control characteristic would have approximately twice the slope of the individual control characteristic 501. The magnitude of the AM characteristic would be approximately double the individual AM characteristic 503. The increased AM characteristic is undesirable, because the AM modulation would result in power fluctuations on the RF output signal 313.
The lossy elements were not present in this circuit, thus, allowing for a maximum potential gain and efficiency of the amplifier. However, the isolation of the RF signal from the amplifier in absence of bias current is dependent on the parasitic capacitance of the transistor 309. Thus, the power control range is not maximized.
Therefore, there is a need for a temperature stable, power efficient RF power amplifier having a large power control range, and minimal AM output.